The objectives of the proposed research are to elucidate the underlying mechanisms of transport and selective membrane permeability in secretory epithelia. Although past research efforts have focused primarily on numerous model systems in reabsorptive epithelia, the paucity of appropriate model systems in secretory epithelia has greatly limited our knowledge of ion transport in this class of transporting epithelia. The model system described in this proposal, the teleost opercular epithelium, offers the distinct advantages of providing ready access to mucosal and serosal compartments and application of the short-circuit current technique (including ion fluxes). These studies will combine morphological and electrophysiological techniques such as ultrastructural cytochemical localization of Na, K-ATPase, fine structural visualization of electron opaque tracers, and measurement of key transepithelial electrical properties and ion fluxes. Particular attention is directed on the following basic transport processes: (a) the morphology of possible paracellular pathways (zonulae occludentes) for transepithelial ion permeation; (b) the controversial mechanism of sodium transport in seawater teleosts; (c) the role of Na, K-ATPase in chloride secretion; and (d) the roles of selected hormones in the mediation of secretory mechanisms.